1. Field of the Invention
The present invention relates to image forming apparatuses that form visible images by electrophotography, such as copiers, printers, recorded image display apparatuses, and facsimile machines.
2. Description of the Related Art
Electrophotographic image forming apparatuses are widely used for copiers, multifunction devices having a printing function, and facsimile machines. In recent years, electrophotographic full-color image forming apparatuses have become very popular.
As color image reproduction with full-color image forming apparatuses becomes popular, improvement in color reproduction range and improvement in image quality by reducing graininess have been desired.
From a different point of view, a printing method has been becoming popular in recent years, with which an image is provided with a photographic finish by adding a gloss coating over the entire image or a new expression different from color is produced by changing the gloss of the image.
In order to realize such expression with an electrophotographic image forming apparatus, image forming methods using clear toner have been proposed, among others.
For example, Japanese Patent Laid-Open No. 2006-251717 describes a structure including four color developing units for forming toner images in four respective colors and a developing unit for forming a clear toner image. The document also describes a structure (tandem structure) in which a plurality of image forming units each including a photo conductor and a corresponding developing unit are arranged in a direction in which an intermediate transfer member moves. The document also describes a structure (rotary structure) in which a photo conductor is disposed and a plurality of developing units are moved to a developing position. The rotary structure is superior to the tandem structure in that a smaller image forming apparatus can be easily made.
However, the rotary structure has the following problem. For example, when a developing unit performs developing, air movement is generated by rotation of the image bearing member. The air movement is in the rotation direction of the image bearing member. The air movement causes toner to be scattered from the developing sleeve in the rotation direction of the image bearing member. Therefore, among the developing units adjacent to the developing position, a developing unit in the scattering direction of the toner is liable to color mixing because the scattered toner may enter the developing unit.
In particular, a developing unit using clear toner is more sensitive to toner mixing than development units using color toner. That is because, when color toner is mixed in clear toner, the color toner is very conspicuous even if the amount of the color toner is very small.
The same is true for white toner as in clear toner. Moreover, the same is true for light toner, when dark and light toner with the same hue and different brightnesses are used.